Add all three fractions together to get the total growth in 3 weeks.
2 1/2+2 3/4+3 1/4=x (x being how much the sunflower grew in the 3 weeks)
Convert all fractions so they are mixed numbers.
5/2+11/4+13/4=x
11/4+13/4=24/4, or 6
Convert 6 to the fraction 12/2.
5/2+12/2=17/2, or 8 1/2
Hope this helps :)